As electronic devices are integrated into smaller portable packages, there is an increasing need for a battery power source that can allow a practical operation period of the device. In most cases, economics require such batteries to be rechargeable. Unfortunately, the technology concerned with energy density of such power sources has not kept pace with electronics integration technology. This has resulted in the now common situation where the battery constitutes a major portion of the volume and weight of a given system, examples of which are cellular phones and portable computers.
A solution, in part, lies not in a breakthrough battery chemistry, but in the understanding of the paradigm which dictates that batteries must last as long as possible. The reason is the long recharge period of the battery. Currently most battery rechargers take from one to three hours to completely recharge a battery. This is referred to as a fast or rapid charge. However, this is too long to wait considering the device it powers may only operate one to three hours on a full charge. If the battery could be recharged in a much shorter period of time, 10-15 minutes for example, then, at least in some cases, consumers would not need large batteries or multiple batteries since the battery could be fully recharged during a coffee break, or while traveling to a business appointment.
The technology necessary to recharge batteries faster has existed, though this fast recharge is often at the expense of cycle life. That is, the number of charge/recharge cycles the battery can provide is often degraded by very fast charging. This trade off is acceptable in commercial applications where the cost is passed on to the consumer. However, a consumer who owns a cellular phone wants to avoid buying new batteries as they are somewhat costly. Therefore, a battery power system with long cycle life has a consumer market advantage, and a system that offers a very short recharge time in addition would be very desirable.
The ability to recharge in a short period of time, without substantially degrading cycle life has in fact been achieved, and is referred to as "ultrafast" charging. Several charge regimes have been developed for nickel cadmium and nickel metal hydride battery systems that provide ultrafast charging. All of the ultrafast charge regimes involve the use of a high average current. In fact, for a 15 minute recharge, the average current applied to the battery must be equal to, if not greater than, four times the current level required for a one hour recharge. This is simple in principle, but much more difficult to implement. The problem is that battery packs for consumer use, and many for commercial use, employ a current interrupt device (CID), such as a resettable fuse, such as that sold under the trade name PolySwitch by the Raychem corporation. The CID is responsive to the current level of the battery and disconnects the cells from further conduction once some threshold current has been reached. This prevents accidental shorting of the battery pack which could otherwise pose a risk to the battery. Hence removal of the CID is not a desirable option.
Typically the safety threshold current level is near, or lower than the ultrafast recharge current level. When these battery packs are recharged at such a high current level, the CID activates and disconnects the battery pack from further charging. It is possible to have a battery pack with separate charging and device contacts, thereby allowing placement of the CID between the battery cells and the device contacts. This approach requires protection of the charger contacts with a diode to prevent an accidental discharge through that path. Two problems exist with this approach. First, in small multiple cell batteries, where the components are packaged very tightly to minimize the size of the battery pack, there is the risk of internal shorting, that is, from one cell to another, or even across the entire cell pack. The occurrence of such an event is minimized by the design of the battery pack, but, once sold, it may experience any number of abusive conditions including being crushed, punctured, or excessively vibrated. By placing the CID in between the cells, the safety of the battery pack is improved since only a limited number of cells could short together. For example, in a six cell battery, the maximum number of cells that could be shorted can be reduced from 6 to 3 by placing the CID in the middle of the cell pack.
The second problem results from the use of a diode in the recharge current path. Given the high current used in ultrafast charging, the diode will heat and dissipate that heat inside the battery pack. In some cases this can cause the charger to stop charging prematurely. In addition, some regimes require a brief discharge pulse at regular intervals during the charge process. This is commonly known as "burp" charging since it helps to eliminate the formation of gases inside the battery cells.
Therefore there exists a need for a battery pack with an optimally placed CID that can be recharged in an ultrafast manner. When there are separate charger and device contacts, there is also a need to provide discharge protection for the charger contacts when the battery pack is not in a charger, but may allow discharge when being charged. Additionally, it would be marketably advantageous if such a battery pack was retrofittable into existing battery chargers.